A method for forming a multilayer coating film by a 3-coat-2-bake (3C2B) process is widely used as a method for forming a coating film on automobile bodies. This method comprises the following steps after applying electrodeposition coating to a substrate: application of an intermediate coating composition→curing by baking→application of an aqueous base coating composition→preheating (preliminary heating)→application of a clear coating composition→curing by baking. However, in recent years, for the purpose of saving energy, attempts have been made to omit the bake-curing step that is performed after applying the intermediate coating composition and use a 3-coat-1-bake (3C1B) process comprising the following steps after applying electrodeposition coating to a substrate: application of an aqueous intermediate coating composition→preheating (preliminary heating)→application of an aqueous base coating composition→preheating (preliminary heating→application of a clear coating composition→curing by baking (for example, Patent Literature 1).
In view of controlling the environmental pollution caused by the vaporization of organic solvents, the establishment of a 3-coat-1 bake process using aqueous coating compositions as the intermediate coating composition and the base coating composition is particularly desired.
The 3-coat-1-bake process using an aqueous intermediate composition and an aqueous base coating composition has the following drawback due to the use of water as a main solvent in the composition. When an aqueous base coating composition is applied to an intermediate coating film, the intermediate coating film is dissolved by the water contained in the aqueous base coating composition, thus forming a mixed layer at the interface between the intermediate and base coating films and resulting in a coating film having low smoothness, low distinctness of image, etc. Accordingly, it is considered to be effective to reduce the water absorption rate of the intermediate coating film (Patent Literature 1).
However, simply adjusting the water absorption rate of the intermediate coating film is not enough. In some cases, disadvantageously, a solvent contained in the clear coating composition permeates and swells the intermediate coating film and the base coating film, causing the formation of minute surface roughness; and the resulting multilayer coating film has a poor finished appearance due to reduced smoothness and distinctness of image.
Further, the coating layer formed by the above conventional method was also insufficient in terms of chipping resistance.